This invention relates to a borehole surveying system and more particularly to a method and apparatus for continuously surveying a borehole drilled substantially horizontally throughout its length.
Drilling holes directionally and using surveying tools to provide directional information for such drilling has become a well-developed art. For the most part, however, this work has been in conjunction with conventionally drilled oil and gas wells with the critical directional aspects being associated with a particular phase of the drilling such as the beginning or end of the drilling operation. An example of this work has been the extensive use of directionally drilled holes in offshore operations. In recent years, a new technology has begun to develop around a method of drilling holes under rivers or the like to facilitate the passage of pipelines. River crossings have historically been a weak link in the construction of cross-country pipelines. Before the use of underground river crossings for pipelines, the two traditional methods were using bridges and laying pipe in dredged channels. There are several problems associated with these prior techniques, namely, interruption of navigation, danger to marine life, time consuming application for permits, costly river bank restoration and continuing river bank maintenance. Now, with the directionally-controlled horizontal drilling process, pipelines are run deep enough below river beds so that future erosion or scouring cannot expose them to the elements nor will dredging equipment or the like disturb them. During installation, river traffic on navigable streams and rivers is not disrupted. Since very little earth is moved (only that which is actually drilled from the hole), riverbeds and banks are left unaltered. Drilled crossings can also be made in highly congested areas utilizing existing corridors. River crossing systems as described above are shown in U.S. Pat. Nos. 3,878,903; 3,894,402; 4,003,440; 4,051,911 and 4,078,617. These operations are conducted using a custom built drilling rig which can drill up to 30 degrees from the horizontal along a predetermined profile under waterways or other obstructions to a targeted surface area on the opposite side of the riverbank or other obstruction. Safety and maintenance is greatly improved with the use of underground river crossings over prior techniques. Statistics show that pipeline river crossing failure due to natural or other causes, occurs approximately 30 times more frequently than failure in the cross-country portion of a pipeline. River dredging and cutting the banks for approaches can result in major earth moving operations. Erosion of the riverbed or recession of the river's bank can result in the exposure and failure of buried pipeline. Drilled crossings do not disturb the river or river bottom and in the case of waterways which are deepened and widened, placing the pipelines 25' to 100' under the existing bottom ensures the safety of the pipeline beyond any normal amount of exposure from natural or man-made activities.
Pipe laying has always presented special environmental problems. Returning the environment to its original state, once disturbed, has become a significant cost factor when designing and constructing a pipeline water crossing. Conservationists have justifiably caused pipeliners to take every consideration in mind when proposing a pipeline project. The drilling of pipeline water crossings eliminates the majority of concerns about these problems, thus making a significant difference in the time needed to secure the permits required to start a project. Because of the minimal effect drilled pipeline crossings have on the environment, fewer permits are required and are generally easier to obtain.
Thus directionally drilled water crossings provide the following advantages: (a) not being subject to damage from erosion and dredging operations, (b) construction schedules not subject to waterway conditions such as rising water, (c) permits are easier and faster to obtain because of lessened environmental impact, (d) waterway traffic is not exposed to navigational hazards or interruptions, (e) the cost of moving vast amounts of earth is eliminated as well as restoration costs of banks and future maintenance, (f) drilled river crossings are virtually maintenance free, thus eliminating a large expense in pipeline maintenance, and (g) drilling can be done in congested corridors and near existing pipelines.
The success of drilled river crossings has led to an enlarged concept for the use of drilled pipeline passages. For example, pipelines constructed from offshore areas often encounter physical barriers or obstacles as they come ashore. Obstacles encountered include the difficulties to construct through heavy surf or unstable shore areas, such as unstable slope or scour conditions, environmental or recreational restrictions, construction difficulties from steep bluffs, man-made obstructions such as roadways and canals, and congested industrial or commercial areas making above ground right-of-way difficult to obtain and hazardous. These considerations are set forth in detail in a technical paper, Number OTC 3786, entitled "Overcoming Obstacles in Landing Pipelines" by Hugh W. O'Donnel and presented at the Offshore Technology Conference in Houston, Tex., May 5-8, 1980.
In the past, guidance of the above described drilling technique has been accomplished using a bent sub in the drill string and periodically running a single shot directional recording instrument together with other conventional surveying tools to intermittently determine the course of the hole being drilled. Because of the nature of this operation, a fairly steady path need be drilled, i.e., without dog legs to permit the easy running in of pipe into the hole. (See prior art patents listed above for details of this technique.) This need for precise directional control requires the frequent running of single shot recording surveys, thus presenting a time-consuming and costly procedure. It is therefore an object of the present invention to provide a new and improved method and apparatus for continuously conducting controlled directional drilling of substantially horizontal boreholes.